docs: def set_priority(self, p_priority):
          """
          Sets the priority of the todo. Must be a single capital letter [A-Z],
          or None to unset the priority.
          Priority remains unchanged when an invalid priority is given, or when
          the task was completed.
          """
          if not self.is_completed() and (p_priority is None or
                                          is_valid_priority(p_priority)):
              self.fields['priority'] = p_priority
  
              priority_str = '' if p_priority is None else '(' + p_priority + ') '
              self.src = re.sub(r'^(\([A-Z]\) )?', priority_str, self.src)